exit(3): make it thread-safe

It was explained by Rich Felker <dalias@libc.org> on libc-coord.
See https://austingroupbugs.net/view.php?id=1845.

Reviewed by: imp, markj
Tested by: antoine (exp-run)
Sponsored by: The FreeBSD Foundation
MFC after: 1 month
Differential revision: https://reviews.freebsd.org/D46108